1. Field of the Invention
The present invention relates to a semiconductor optical device.
2. Related Background Art
Semiconductor lasers have been known as one of semiconductor optical devices. For example, as semiconductor lasers having emission wavelengths in a wavelength band of at least 0.9 μm but not more than 1.25 μm, those equipped with a semiconductor layer of GaInAs, GaInAsP, or the like on a GaAs substrate have been known. In particular, semiconductor lasers having an oscillation wavelength in a 0.98-μm band have been in use as pumping light sources for erbium-doped optical fiber amplifier s (hereinafter abbreviated as “EDFA”) which are optical amplifiers in a 1.55-μm wavelength band. Semiconductor lasers having emission wavelengths in a 1.017-μm band have been in use as pumping light sources for praseodymium-doped optical fiber amplifiers (hereinafter abbreviated as “PDFA”) which are optical amplifiers in a 1.3-μm wavelength band.
A semiconductor laser fabricated on a GaAs substrate is described in IEEE J. Quantum Electron., vol. 36, no. 12, (2000), pp. 1454-1461, and International Publication No. 2004/027950 pamphlet. This semiconductor laser has specific wavelength characteristics, especially wavelength fluctuations, due to a resonance coupling between a waveguide mode and a GaAs substrate mode. To reduce the resonance coupling between the waveguide mode and the GaAs substrate mode, a semiconductor laser having a light absorbing layer between the GaAs substrate and the lower electrode is disclosed in Japanese Patent Application No. 2001-144371.